High intensity focused ultrasound (HIFU) has been used as a non-invasive precise treatment modality for internal pathological conditions such as tumors and abnormal vascular or nerve conditions. While diagnostic ultrasound has a focal intensity typically around 0.1 W/cm2, high intensity focused ultrasound is of 4-5 orders of magnitude greater in focal intensity, typically in the range of 1,000 to 10,000 W/cm2. HIFU energy, focused at locations deep in tissue, leaves the intervening tissue between the HIFU source and the focus unharmed. At the HIFU focus, however, the focal temperature may quickly exceed 70° C., and thereafter reach 100° C., the boiling point of tissue water, depending on the application of the HIFU energy. The high focal tissue temperature generated by the HIFU energy can rapidly cause tissue disruption. The thermal effect of tissue destruction is augmented further by the mechanical effect of HIFU energy. The combined thermal and mechanical effect at the tissue focus of the HIFU is being used for the treatment of uterine fibroid tumors, prostate hyperplasia or cancer, liver cancer, malignant bone and soft tissue sarcoma and internal bleeding.
Since HIFU treatment is mostly directed to internal pathological conditions, which cannot be visually seen, the use of radiologic imaging of those pathologies deep in the tissue is necessary for the therapy. MRI is being used to guide HIFU treatment of internal fibroids. Transabdominal ultrasound-guided HIFU treatment of liver tumors and uterine fibroids is also being practiced.
Recently, transvaginal ultrasound image-guided HIFU treatment of uterine fibroids has been developed. Similar technology can be applied to endometrial ablation and treatment of cervical neoplasia and HPV lesions. In order to properly treat the deep uterine pathologies, such as fibroids, it is preferred that the tumor along with the surrounding uterine tissue be visualized in real time throughout the HIFU treatment process. Both clear imaging by diagnostic ultrasound and achievement of HIFU tissue effect at the target area are important when conducting image-guided HIFU therapy.
In order to use ultrasound energy to image the structures of an area for treatment, the imaging scan head traditionally has to be in direct and firm contact with the tissue in continuum to the tissue of the target area. This engagement of the imaging scan head to the tissue may be supplemented by a coupling medium which can effectively transmit the ultrasound between the scan head and the tissue. For example, ultrasound gel is traditionally used to couple an ultrasound scan head and the skin on a person's abdomen to visualize intra-abdominal structures. The coupling material, such as ultrasound gel, is of similar acoustic transmission characteristics as that of the tissue to prevent an acoustic aberrance at the scan head-tissue interface. For example, if there is air or other obstructions between the scan head and the skin, the ultrasound imaging will become distorted or non-observable due to the difference of acoustic impedance of the air or other obstructions from that of the tissue. The ultrasound gel as a coupling medium replaces the air at the interface and enables clearer imaging of the underlying structures.
A conventional HIFU transducer generating therapeutic ultrasound energy likewise should be in direct engagement of the tissue in continuum with the target in order for the ultrasound energy to be effectively transmitted and focused at the target area to achieve the therapeutic effect. Generally, a coupling medium of similar acoustic characteristics as the tissue is used to connect the HIFU transducer with the tissue to enable optimal transmission and focusing of HIFU energy. Disengagement of the imaging scan head or the HIFU aperture from the tissue without a mechanism of coupling tends to interfere with the image-guided HIFU treatment of the target tissue.
For example, performing transvaginal ultrasound image-guided HIFU treatment of uterine fibroids requires a physical contact to obtain a proper engagement and coupling of an imaging probe and a HIFU transducer to the cervix and vaginal fornices. The imaging scan head needs to be placed firmly against the cervix and is generally pushed up towards the top of the anterior fornix to obtain optimal ultrasound images of the pelvic organs. Ultrasound gel is used to enhance the coupling between the scan head and tissue. Disengagement between the scan head and the cervix-fornices typically results in poor image quality. The HIFU transducer, which may be in a fixed relationship to the imaging head, engages the cervix, mostly towards the posterior fornix. Due to its size, the HIFU transducer also typically partially rides on the surface of the cervix that has the cervical os in its center. A fixed spatial relationship between the imaging head and the HIFU aperture presents an obvious challenge: optimal engagement of the imaging head with the tissue at the cervix may disengage the HIFU transducer from the cervical tissue toward the posterior fornix and vice versa. The variability of the dimensions and shape of the cervix and vaginal fornices among women makes it very difficult to design a probe that can optimize the simultaneous engagement of both the imaging and HIFU heads to the cervix and vaginal fornices. As noted earlier, when using conventional ultrasound systems, disengagement of the imaging head from the cervical tissue results in poor images of the pelvic organs. Disengagement of the HIFU transducer from the tissue toward the posterior fornix results in intervening air space that can cause aberration of the HIFU effect and even undesirable local heating at the tissue interface.
Thus, there is a need to provide consistent clear imaging of the target tissue and the HIFU effect at the target tissue to help guide movement of the HIFU focus throughout the procedure. Furthermore, there is a need for a more global approach for the engagement and coupling of both an imaging component and HIFU component to the tissue in a body cavity. These needs and other shortcomings in the prior art are addressed herein.